FIG. 3 depicts a conventional protection circuit topology 200. The circuit 200 includes conventional battery charging circuit 20 and a protection circuit formed by a diode 204 and resistor 202 between the battery 40 and the adapter source 22. The battery charging components are well understood in the battery charging art, and include a charger controller 30, a Buck converter represented by the switches 26 and 28, the inductor 32 and the capacitor 34, and a reverse current protection switch 38. The charger 30 controls the Buck converter's switches 26 and 28 using conventional PWM techniques to generate a charging current to the battery 40. The current source is typically an adapter 22. The charger controller controls the amount of current to the battery using feedback information from sense resistors R1 (24) and R2 (36). Sense resistor R1 generates a feedback signal indicative of the amount of current drawn from the adapter 22. Sense resistor R2 generates a feedback signal indicative of the amount of charging current generated by the Buck converter. This feedback data is used to control the duty cycle, and thus the charging current, generated by the charger.
In operation, the charger controller adjusts the duty cycle (PWM) of the switches 26 and 28 to adjust the amount of charging current delivered to the battery. The operation of the charging components 20 are described in further detail in U.S. Patent application Ser. No. 09/948,828 (now U.S. Pat. No. 6,498,461) and U.S. Pat. No. 6,459,602, all of which are incorporated by reference in their entirety as teaching these conventional components and variations thereof.
When the battery disconnects, the charging path (defined between the adapter and the battery) is an open circuit. The limit resistor 202 and diode 204 operate to increase the voltage at the voltage feedback pin (VFB) of the charger, because switch 38 is opened by a control signal generated by the controller 30. As a result, the charger will decrease the duty cycle, and the voltage on the capacitor 34 decreases. When the battery reconnects, the voltage on the capacitor 34 is smaller than the voltage across the battery, and therefore the body diode of switch 38 is reverse biased. In this situation, inrush current into the battery is reduced or eliminated, and the duty cycle of the controller slowly increases to continue charging the battery.
However, this topology does not include a mechanism to prevent or reduce an over-voltage condition at the battery terminals.